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Backscattering Lab

Backscattering Lab. Julia Uitz Pauline Stephen Wayne Slade Eric Rehm. Wetlabs EcoVSF. Samples the Volume Scattering Function (VSF) at three angles 100°, 125 °, and 150° One wavelength: 660 nm for our model is safely in absorbing part of H 2 O spectrum

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Backscattering Lab

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  1. Backscattering Lab Julia Uitz Pauline Stephen Wayne Slade Eric Rehm

  2. Wetlabs EcoVSF • Samples the Volume Scattering Function (VSF) at three angles • 100°, 125 °, and 150° • One wavelength: 660 nm for our model is safely in absorbing part of H2O spectrum • Integrate curve fit of VSF samples from 90 to 180 degrees to compute backscattering coefficient bb. • Employs three transmitters coupled to a single receiver

  3. Backscattering Coefficient bb • bb carries useful information about seawater constituents • Potential to derive information about • Abundance and types of suspended marine particles • Such particles play different roles in ocean ecosystems and biogeochemical cycling • A proxy for particle abundance • Also depends significantly on particle size distribution and particle composition: size, index of refraction, absorption • Smaller particles scatter more • Particles with index of refraction higher than water scatter more • Particles that are highly absorbing scatter (e.g., water filled phytoplankton) scatter less, but in absence of inorganic scatters, can be seen in backscatter. • bb is proportional to spectral reflectance of the ocean (aka “ocean color”). • Understanding bb is required to interpret ocean color

  4. ECO-VSF Calibration • Dark Counts • Factory: 31.047, 30.488, 158.093 • Lab: 29 29 140 • At 150, we have a lower count value • Was our room darker than Wetlabs’? • DI Water • Factory: 39.212, 43.364, 196.515 • Lab: 40 41 144 • At 150, we have lower count value. • Discussion • 150 light source or detector could have changed since factory calibration. Note that blue and red reference values were not output by this EcoVSF. • Our water could be cleaner than Wetlabs’ • Or, since small particles scatter a larger angles, may suggest that the fraction of small particles in their DI water is greater than ours.

  5. Beads DI Water

  6. Why use the factory calibration instead of trusting our own? • Good question… • We should have trusted our calibration and used those dark counts and slopes • In the original presentation subsequent plots used factory calibration • Updated slides will use our calibration • We did as good a job as Wetlab at calibration…

  7. Corrected b

  8. Effect of Absorption Correction Effect on bbp after integration:

  9. VSF for Beads Same particle  Same shape of VSF

  10. VSF for Samples • Filtered sea water scatters at angles larger than other samples • Mean size of Dytilum ~ mean size of total sea water from LISST measurements

  11. bbpvia two methods • bbp from b(100) best matches bbp estimate from all three angles • Overall, very good correlation between methods

  12. Backscattering Ratio bbp:bp

  13. Discussion • Backscattering ratio for dock sample (.0077) is in published range for Case I and Case II waters (Twardowski, et al., JGR, 2001) • Case I: .006 – .020 • Case II: .005 – .013 • Particle Size distribution for dock sample (calculated from AC-9 cp) is in “typical” published range (3.5< x<4) • As we move from less scattering (Dytilum) through scattering (Sea water) to highly scattering (beads), increases from .5% to 1.1%

  14. What can we say about Dytilum brightwellii? • Backscattering ratio • Lower than for unfiltered seawater and homogenous concentrations of 10 µm non-absorbing beads • Highly absorbing and large: D=25-100 µm • Shape of b: • Monotonically decreases between 100 and 150 • Magnitude of b: • ~1 order of magnitude (.0004 - .0002) less than that for unfiltered sea water (.002 - .0014) • PSD inferred from cp : • Larger fraction of large particles than sea water. (x = 3.7 vs. 4.0)

  15. What can we say about Dytilum brightwellii? ~20-60 mm

  16. Unfiltered Sea WaterComparison with LISST ~6-70 mm

  17. What can we say about Dytilum brightwellii?

  18. Eric was confused about EcoVSF • What do you do with it if you don’t own an AC-9 and your measurements are in-situ? • No a No absorption correction for b ~O(1%) error • No b No backscattering ratio • No ap, bp No ap:bpproxy for pigmented material (Twardowski et al., 2001) • No other data on PSD • No cp No g • No Coulter counter

  19. There is some hope…Case I waters, Global Scale(Behrenfeld, 2004) Note: I cut out 8 of Behrenfeld’s 14 steps…. • cp is dominated by particles in the phytoplankton domain • cp covaries with POC (7 references) • cp:chl should track phytoplankton Carbon:chl • (cp:chl tracks changes in phytoplankton physiology like photosynthetic rate) • “Mie calculations indicate that bbpis dominated by submicron particles, but in field populations bbp likely has a significant tail in the phytoplankton size domain.” • Satellite bbp covaries with POC (2 references) • (Should be true in-situ too…) • chl:bbpshould track chl:Carbon and thus phytoplankton growth rates {once a correction for bacterial background is accounted for}

  20. Peak in chl, bb and chl:bb Pressure (dbar) Raw ECO-VSF counts

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